Heretofore, a method for manufacturing a three-dimensional shaped object by irradiating a powder with a light beam has been known (such method can be generally referred to as selective laser sintering method or metal laser-sintering). Such method can produce a three-dimensional shaped object with a plurality of solidified layers stacked integrally by repeating the step (i) of forming a solidified layer by irradiating a predetermined portion of a powder layer with a light beam, thereby allowing sintering of the predetermined portion of the powder or melting and subsequent solidification thereof, and the step (ii) of forming another solidified layer by newly forming a powder layer on the resulting solidified layer, followed by similarly irradiating the powder layer with the light beam (see JP-T-01-502890 or JP-A-2000-73108). The three-dimensional shaped object thus obtained can be used as a metal mold in a case where a metal powder is used as the powder material. While on the other hand, the three-dimensional shaped object can be used as a plastic replica in a case where a resin powder is used as the powder material. This kind of technology makes it possible to produce the three-dimensional shaped object with a complicated contour shape in a short period of time. Particularly when a sufficient melting of the metal powder occurs by the irradiation of the light beam with high energy density, a sintered density of almost 100% can be achieved after the solidification of the melted metal powder. The resulting shaped object with such high density can be subsequently subject to a finish machining treatment for smoothing a surface thereof. The shaped object thus finally obtained can be used as a metal mold for plastic molding.
In this regard, however, the metal powder used as a raw material for such selective laser-sintering is required to have different characteristics from those of another kind of powder-sintering process in which a powder compacting followed by a sintering of the compacted powder is performed.
For example, it is required for the metal powder to have a particle diameter smaller than the thickness of the powder layer to be irradiated with the light beam. Such smaller particle diameter provides a higher packing density of the powder as well as an improved absorption efficiency of the light beam upon producing the shaped object. This will lead to a higher sintered density and a smaller surface-roughness of the shaped object. On the other hand, when the particle diameter is too small, the metal powder tends to form the aggregated particles so that a packing density of the powder becomes lower, thus making it impossible to uniformly form a thin metal layer thereof.
In order to increase the strength of the shaped object, it is required that a contact area is large and a bonding strength is high between a newly formed sintered layer and a preceding and solidified sintered layer lying thereunder. In this case, even between the newly formed sintered layer and an adjacent solidified sintered layer, there is required a large contact area and a high bonding strength.
Furthermore, it is required in the selective laser sintering method that a top surface of the newly formed sintered layer does not have a significant bulge. The bulge with more than the thickness of the powder layer can interfere with the spreading of the subsequent powder layer, making it impossible to suitably form such subsequent powder layer.
Upon irradiating the metal powder with the light beam, the metal powder is allowed to melt partially or wholly. The melted metal powder is then solidified through a subsequent rapid cooling thereof. This results in a formation of a sintered material. When the melted metal powder has a high wettability, the contact area between the melted metal powder and the adjacent sintered layer becomes larger, in which case a less bulge is formed if the melted metal powder has a higher fluidity. Therefore, it is desired that the metal powder, when melted, has not only a high wettability but also a high fluidity.